Hmmmm.. Because using your operating systemīs theme as app skin is something that is taught in every single UX-desinging class. Apps that seemingly go with main operating systems theme are regarded as something that every single devel should be encouraged to do :| For more about this follow recent market speeches about windows mobile and how user canīt see where apps start and where OS ends. I do not totally agree or like this but to me itīs something that seems to be something that most UI designers want.